In this study, a 3D simple shear model using DEM is built based on the boundary condition of an NGI-type bidirectional simple shear apparatus. Stack of rings used as lateral constraints in a bidirectional simple shear test is modelled by layers of clumps which is possible to be moved by particles; different contact types and parameters are used to model the sand-loading caps, sand-latex membrane, and sand-sand contacts. A simple shear test using the bidirectional simple shear apparatus is performed for the calibration of the 3D DEM simple shear model. By analyzing the simulation results, the following can be concluded. (1) Rings generated by clumps can provide an accurate boundary condition, effective in computation since no contact force is needed for a clump. (2) In the simulation, the orientation of average contact force changed dramatically during shear. It is in the vertical direction (90°) before shear and changes to 45° at 40% shear strain. No shear band is observed which is consistent with the test, and particles move uniformly. (3) In the simulation, the degree of noncoaxiality is the greatest at the beginning of shear, and it is decreased during shear. However, the degree of noncoaxiality is still large at 20% shear strain where there is a 10° difference between the rotation angle of principal stress and principal strain increment.

中文翻译：

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3D DEM简单剪切模型的改进边界条件

在这项研究中，基于NGI型双向简单剪切仪的边界条件，建立了使用DEM的3D简单剪切模型。在双向简单剪切试验中用作侧向约束的环的堆叠是通过成团的层进行建模的，成团的层可能会被颗粒移动。使用不同的接触类型和参数来模拟装砂帽，乳胶膜和沙砂接触。使用双向简单剪切设备执行简单剪切测试，以校准3D DEM简单剪切模型。通过分析仿真结果，可以得出以下结论。（1）由团块产生的环可以提供精确的边界条件，由于团块不需要接触力，因此在计算中很有效。（2）在模拟中，在剪切过程中，平均接触力的方向发生了巨大变化。它在剪切之前是在垂直方向（90°）上，并在40％剪切应变下变为45°。没有观察到与试验一致的剪切带，并且颗粒均匀移动。（3）在模拟中，非同轴度在剪切开始时最大，在剪切过程中减小。但是，在20％的剪切应变下，非同轴度仍然很大，此时主应力的旋转角度和主应变增量之间存在10°的差异。并在剪切过程中减少。但是，在20％的剪切应变下，非同轴度仍然很大，此时主应力的旋转角度和主应变增量之间存在10°的差异。并在剪切过程中减少。但是，在20％的剪切应变下，非同轴度仍然很大，此时主应力的旋转角度和主应变增量之间存在10°的差异。